1. Field of the Invention
The present invention relates to an image forming apparatus and a method of creating evaluation data.
2. Description of Related Art
An electrophotographic image forming apparatus forms a reference image on a sheet and reads and analyses the reference image to evaluate image quality for stabilizing image quality.
When a reading section disposed above a sheet conveyance path reads the reference image, high-speed reading of the reference image is expected in connection with the speed-up of image formation.
A method of achieving high-speed reading is to reduce the resolution at the time of reading. Unfortunately, however, if the reference image is read at a resolution lower than the resolution at which the reference image is formed, some evaluation items, such as line reproducibility, are difficult to evaluate accurately if the reference image does not have a high resolution.
A method of obtaining a high-resolution read image using a low-resolution reading section was proposed (see, for example, Japanese Unexamined Patent Application Publication No.6-164849). Specifically, the method obtains a high-resolution image by reading an image while finely shifting its reading position to increase the number of read values for each pixel.
From the same point of view, a method of reading a reference image while tilting the image was also proposed (see, for example, Japanese Unexamined Patent Application Publication No. 6-186000).
Unfortunately, however, in the case of reading an image while shifting its reading position, the frequency of reading is increased. This fails to achieve high-speed reading. In addition, shifting a reading position with high accuracy is difficult, and improper reading positions lead to lower reliability of read values.
In the case of reading a reference image while tilting it, the sheet on which a reference image has been formed needs to be tilted at the time of the reading. Tilting a sheet tends to cause deformation, such as wrinkle and curling, of the sheet. This may reduce the reading accuracy, in fact.